Many of major autoimmune diseases are sexually dimorphic. Sytemic Lupus Erythematosus (SLE), scleroderma, multiple sclerosis, Sjogren's syndrome, autoimmune diseases of thyroid gland and others are primarily occurring in females. In a mouse model of spontaneous type 1 diabetes (T1D in non obese diabetic, NOD, mice) the disease is stronger in females. A survey of NOD colonies around the world found that not only overall incidence of T1D varied greatly between different facilities, but that the female/male incidence ratio varied. The variability suggests that some factors that cannot be easily accounted for clearly affect the disease progression. In other facilities, including ours, the incidence in males is even lower (about 15-20% at 30 wks), whereas the incidence in females is around 65-80%. As a part of the studies of the role of microbial environment on T1D development, we have rederived NOD mice into germ-free (sterile) conditions. We realized that these mice no longer showed sexual dimorphism! That means that the hormones (clearly shown to be involved) and the microbiota are interacting in some way that affects the disease progression. Our main hypothesis suggests that the hormones and microbial products act together (dual-signal hypothesis) to induce tolerance to T1D in males. Preliminary analysis has revealed a role for Interferon-gamma in protection of males from T1D. It is proposed to test the hypothesis by a variety of approaches including colonization of germ-free mice with defined microbiota, genetic approach and gene expression analysis. We will also to test whether systemic immunity in a common genetically complex mouse model of SLE is also sexually dimorphic because of the presence of microbes. Accordingly, two specific aims are: Specific Aim 1. Investigate the effector mechanisms that make NOD males more resistant to T1D. We will test the role of specific microbial lineages in controlling sexual dimorphism; We will study the signaling pathway and cell types involved in tolerance induction using gene-expression and genetic approaches. We will study how manipulations of sex hormones affect microbial composition and functions; We will use gene expression analysis to delineate signaling pathways important for hormone-dependent control over microbiota. Specific Aim 3. Investigate whether sexual dimorphism in a mouse model of SLE is dependent on microbes. We will study the role of microbiota in induction and sexual dimorphism in NZBWF1 mice in germ-free environment.